System and method for addressing a MEMS display

ABSTRACT

A system and method for addressing an array of MEMS display elements, such as interferometric modulators, from a drive control. A display includes groups of display elements that are addressed with a commonly applied drive signal. In one embodiment, the groups of display elements are configured to have different response times and are driven by pulses of varying length indicative of those response times. In another embodiment, the groups of display elements are configured to have different actuation voltages and are driven by pulses of varying voltage indicative of those actuation voltages.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.11/051,251, filed Feb. 4, 2005, now U.S. Pat. No. 7,142,346, titled“SYSTEM AND METHOD FOR ADDRESSING A MEMS DISPLAY”, which claims priorityto (1) U.S. Provisional Application No. 60/613,458, titled “PIXELELEMENT WITH SUB-PIXELS HAVING DIFFERING ACTUATION VOLTAGES”, filed Sep.27, 2004; (2) U.S. Provisional Application No. 60/604,896, filed Aug.27, 2004 titled “METHODS OF ADDRESSING A BI-STABLE MODULATOR”; and (3)U.S. Provisional Application No. 60/606,223, filed Aug. 31, 2004, titled“METHODS OF ADDRESSING A BI-STABLE MODULATOR”, and which is also (4) acontinuation in part of U.S. application Ser. No. 10/731,989, filed Dec.9, 2003, now U.S. Pat. No. 7,161,728. Each of the foregoing applicationsis incorporated by reference, in its entirety.

BACKGROUND

1. Field

The field of the invention relates to microelectromechanical systems(MEMS).

2. Description of the Related Technology

Microelectromechanical systems (MEMS) include micro mechanical elements,actuators, and electronics. Micromechanical elements may be createdusing deposition, etching, and or other micromachining processes thatetch away parts of substrates and/or deposited material layers or thatadd layers to form electrical and electromechanical devices. One type ofMEMS device is called an interferometric modulator. An interferometricmodulator may comprise a pair of conductive plates, one or both of whichmay be transparent and/or reflective in whole or part and capable ofrelative motion upon application of an appropriate electrical signal.One plate may comprise a stationary layer deposited on a substrate, theother plate may comprise a metallic membrane separated from thestationary layer by an air gap. Such devices have a wide range ofapplications, and it would be beneficial in the art to utilize and/ormodify the characteristics of these types of devices so that theirfeatures can be exploited in improving existing products and creatingnew products that have not yet been developed.

SUMMARY

The system, method, and devices of the invention each have severalaspects, no single one of which is solely responsible for its desirableattributes. Without limiting the scope of this invention, its moreprominent features will now be discussed briefly. After considering thisdiscussion, and particularly after reading the section entitled“Detailed Description of Preferred Embodiments” one will understand howthe features of this invention provide advantages over other displaydevices.

One embodiment is a light modulator that includes an array of elementsat least some of which have different values of deflection versusaddressing pulse width. The light modulator further includes addressingcircuitry configured to provide addressing pulses of varying width tothe array of elements such that different combinations of elementsswitch in a selectable manner, depending upon the width of theaddressing pulses.

Another embodiment is a light modulator that includes an array ofelements having differing values of deflection versus applied voltage.The light modulator further includes addressing circuitry configured toprovide addressing pulses of varying voltage level to the array ofelements such that different combinations of elements switch in aselectable manner, depending upon the voltage level of the addressingpulses.

Another embodiment is a display including a plurality of MEMS elementsarranged in rows. The MEMS elements of each of the rows are furtherarranged in subrows. The subrows of each row are electrically connected.The display further includes a plurality of resistors. Each of theresistors is connected to a respective one of the subrows. Therespective one of the resistors for each of the subrows of each row hasa different resistance from the resistors connected to the other subrowsof the row.

Another embodiment is a method of addressing a plurality of displayelements having at least a first and second display element andcharacterized by respective response thresholds. The method includesgenerating a first pulse characterized by a parameter having a valuegreater than the response threshold of the first display element andless than the response threshold of the second display element. Themethod further includes applying the first pulse to the plurality ofdisplay elements.

One embodiment is a driver circuit for addressing a plurality of displayelements having at least a first and second display element andcharacterized by respective response thresholds. The driver circuitincludes means for generating a first pulse characterized by a parameterhaving a value greater than the response threshold of the first displayelement and less than the response threshold of the second displayelement. The driver circuit further includes means for applying thefirst pulse to the plurality of display elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view depicting a portion of one embodiment of aninterferometric modulator display in which a movable reflective layer ofa first interferometric modulator is in a released position and amovable reflective layer of a second interferometric modulator is in anactuated position.

FIG. 2 is a system block diagram illustrating one embodiment of anelectronic device incorporating a 3×3 interferometric modulator display.

FIG. 3 is a diagram of movable mirror position versus applied voltagefor one exemplary embodiment of an interferometric modulator of FIG. 1.

FIG. 4 is an illustration of a set of row and column voltages that maybe used to drive an interferometric modulator display.

FIGS. 5A and 5B illustrate one exemplary timing diagram for row andcolumn signals that may be used to write a frame of display data to the3×3 interferometric modulator display of FIG. 3.

FIG. 6A is a cross section of the device of FIG. 1.

FIG. 6B is a cross section of an alternative embodiment of aninterferometric modulator.

FIG. 6C is a cross section of another alternative embodiment of aninterferometric modulator.

FIG. 7 is a partial schematic diagram of an embodiment of aninterferometric modulator display in which the rows have been subdividedinto three subrows that share a common driver connection.

FIG. 8 is a timing diagram that illustrates a series of row and columnsignals applied to the top row of the embodiment of the array of FIG. 7to produce the illustrated display arrangement.

FIG. 9 is a diagram, similar to that of FIG. 3, of movable mirrorposition versus applied positive voltage illustrating an exemplaryembodiment of three interferometric modulators that have nestedstability windows.

FIG. 10 is a timing diagram that illustrates a series of row and columnsignals applied to the top row of the embodiment of the array of FIG.10A to produce the illustrated display arrangement.

FIG. 11 is a flowchart illustrating one embodiment of a method ofdriving an interferometric modulator array such as described withrespect to FIGS. 6 and 7.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In preferred embodiments, the invention addresses a group of displayelements with a drive signal applied through a common driver connectionto the group of display elements. The display can thus produce moreshades of gray, or color, with a smaller number of leads than would benecessary if drive signal for each display element were applied throughseparate leads for each display element.

The following detailed description is directed to certain specificembodiments of the invention. However, the invention can be embodied ina multitude of different ways. In this description, reference is made tothe drawings wherein like parts are designated with like numeralsthroughout. As will be apparent from the following description, theinvention may be implemented in any device that is configured to displayan image, whether in motion (e.g., video) or stationary (e.g., stillimage), and whether textual or pictorial. More particularly, it iscontemplated that the invention may be implemented in or associated witha variety of electronic devices such as, but not limited to, mobiletelephones, wireless devices, personal data assistants (PDAs), hand-heldor portable computers, GPS receivers/navigators, cameras, MP3 players,camcorders, game consoles, wrist watches, clocks, calculators,television monitors, flat panel displays, computer monitors, autodisplays (e.g., odometer display, etc.), cockpit controls and/ordisplays, display of camera views (e.g., display of a rear view camerain a vehicle), electronic photographs, electronic billboards or signs,projectors, architectural structures, packaging, and aestheticstructures (e.g., display of images on a piece of jewelry). MEMS devicesof similar structure to those described herein can also be used innon-display applications such as in electronic switching devices.

One interferometric modulator display embodiment comprising aninterferometric MEMS display element is illustrated in FIG. 1. In thesedevices, the pixels are in either a bright or dark state. In the bright(“on” or “open”) state, the display element reflects a large portion ofincident visible light to a user. When in the dark (“off” or “closed”)state, the display element reflects little incident visible light to theuser. Depending on the embodiment, the light reflectance properties ofthe “on” and “off” states may be reversed. MEMS pixels can be configuredto reflect predominantly at selected colors, allowing for a colordisplay in addition to black and white.

FIG. 1 is an isometric view depicting two adjacent pixels in a series ofpixels of a visual display, wherein each pixel comprises a MEMSinterferometric modulator. In some embodiments, an interferometricmodulator display comprises a row/column array of these interferometricmodulators. Each interferometric modulator includes a pair of reflectivelayers positioned at a variable and controllable distance from eachother to form a resonant optical cavity with at least one variabledimension. In one embodiment, one of the reflective layers may be movedbetween two positions. In the first position, referred to herein as thereleased state, the movable layer is positioned at a relatively largedistance from a fixed partially reflective layer. In the secondposition, the movable layer is positioned more closely adjacent to thepartially reflective layer. Incident light that reflects from the twolayers interferes constructively or destructively depending on theposition of the movable reflective layer, producing either an overallreflective or non-reflective state for each pixel.

The depicted portion of the pixel array in FIG. 1 includes two adjacentinterferometric modulators 12 a and 12 b. In the interferometricmodulator 12 a on the left, a movable and highly reflective layer 14 ais illustrated in a released position at a predetermined distance from afixed partially reflective layer 16 a. In the interferometric modulator12 b on the right, the movable highly reflective layer 14 b isillustrated in an actuated position adjacent to the fixed partiallyreflective layer 16 b.

The fixed layers 16 a, 16 b are electrically conductive, partiallytransparent and partially reflective, and may be fabricated, forexample, by depositing one or more layers each of chromium andindium-tin-oxide onto a transparent substrate 20. The layers arepatterned into parallel strips, and may form row electrodes in a displaydevice as described further below. The movable layers 14 a, 14 b may beformed as a series of parallel strips of a deposited metal layer orlayers (orthogonal to the row electrodes 16 a, 16 b) deposited on top ofposts 18 and an intervening sacrificial material deposited between theposts 18. When the sacrificial material is etched away, the deformablemetal layers are separated from the fixed metal layers by a defined airgap 19. A highly conductive and reflective material such as aluminum maybe used for the deformable layers, and these strips may form columnelectrodes in a display device.

With no applied voltage, the cavity 19 remains between the layers 14 a,16 a and the deformable layer is in a mechanically relaxed state asillustrated by the pixel 12 a in FIG. 1. However, when a potentialdifference is applied to a selected row and column, the capacitor formedat the intersection of the row and column electrodes at thecorresponding pixel becomes charged, and electrostatic forces pull theelectrodes together. If the voltage is high enough, the movable layer isdeformed and is forced against the fixed layer (a dielectric materialwhich is not illustrated in this Figure may be deposited on the fixedlayer to prevent shorting and control the separation distance) asillustrated by the pixel 12 b on the right in FIG. 1. The behavior isthe same regardless of the polarity of the applied potential difference.In this way, row/column actuation that can control the reflective vs.non-reflective pixel states is analogous in many ways to that used inconventional LCD and other display technologies.

FIGS. 2 through 5 illustrate one exemplary process and system for usingan array of interferometric modulators in a display application. FIG. 2is a system block diagram illustrating one embodiment of an electronicdevice that may incorporate aspects of the invention. In the exemplaryembodiment, the electronic device includes a processor 21 which may beany general purpose single- or multi-chip microprocessor such as an ARM,Pentium®, Pentium II®, Pentium III®, Pentium IV®, Pentium® Pro, an 8051,a MIPS®, a Power PC®, an ALPHA®, or any special purpose microprocessorsuch as a digital signal processor, microcontroller, or a programmablegate array. As is conventional in the art, the processor 21 may beconfigured to execute one or more software modules. In addition toexecuting an operating system, the processor may be configured toexecute one or more software applications, including a web browser, atelephone application, an email program, or any other softwareapplication.

In one embodiment, the processor 21 is also configured to communicatewith an array controller 22. In one embodiment, the array controller 22includes a row driver circuit 24 and a column driver circuit 26 thatprovide signals to a pixel array 30. The cross section of the arrayillustrated in FIG. 1 is shown by the lines 1-1 in FIG. 2. For MEMSinterferometric modulators, the row/column actuation protocol may takeadvantage of a hysteresis property of these devices illustrated in FIG.3. It may require, for example, a 10 volt potential difference to causea movable layer to deform from the released state to the actuated state.However, when the voltage is reduced from that value, the movable layermaintains its state as the voltage drops back below 10 volts. In theexemplary embodiment of FIG. 3, the movable layer does not releasecompletely until the voltage drops below 2 volts. There is thus a rangeof voltage, about 3 to 7 V in the example illustrated in FIG. 3, wherethere exists a window of applied voltage within which the device isstable in either the released or actuated state. This is referred toherein as the “hysteresis window” or “stability window.” For a displayarray having the hysteresis characteristics of FIG. 3, the row/columnactuation protocol can be designed such that during row strobing, pixelsin the strobed row that are to be actuated are exposed to a voltagedifference of about 10 volts, and pixels that are to be released areexposed to a voltage difference of close to zero volts. After thestrobe, the pixels are exposed to a steady state voltage difference ofabout 5 volts such that they remain in whatever state the row strobe putthem in. After being written, each pixel sees a potential differencewithin the “stability window” of 3-7 volts in this example. This featuremakes the pixel design illustrated in FIG. 1 stable under the sameapplied voltage conditions in either an actuated or releasedpre-existing state. Since each pixel of the interferometric modulator,whether in the actuated or released state, is essentially a capacitorformed by the fixed and moving reflective layers, this stable state canbe held at a voltage within the hysteresis window with almost no powerdissipation. Essentially no current flows into the pixel if the appliedpotential is fixed.

In typical applications, a display frame may be created by asserting theset of column electrodes in accordance with the desired set of actuatedpixels in the first row. A row pulse is then applied to the row 1electrode, actuating the pixels corresponding to the asserted columnlines. The asserted set of column electrodes is then changed tocorrespond to the desired set of actuated pixels in the second row. Apulse is then applied to the row 2 electrode, actuating the appropriatepixels in row 2 in accordance with the asserted column electrodes. Therow 1 pixels are unaffected by the row 2 pulse, and remain in the statethey were set to during the row 1 pulse. This may be repeated for theentire series of rows in a sequential fashion to produce the frame.Generally, the frames are refreshed and/or updated with new display databy continually repeating this process at some desired number of framesper second. A wide variety of protocols for driving row and columnelectrodes of pixel arrays to produce display frames are also well knownand may be used in conjunction with the present invention.

FIGS. 4 and 5 illustrate one possible actuation protocol for creating adisplay frame on the 3×3 array of FIG. 2. FIG. 4 illustrates a possibleset of column and row voltage levels that may be used for pixelsexhibiting the hysteresis curves of FIG. 3. In the FIG. 4 embodiment,actuating a pixel involves setting the appropriate column to −V_(bias),and the appropriate row to +ΔV, which may correspond to −5 volts and +5volts respectively Releasing the pixel is accomplished by setting theappropriate column to +V_(bias), and the appropriate row to the same+ΔV, producing a zero volt potential difference across the pixel. Inthose rows where the row voltage is held at zero volts, the pixels arestable in whatever state they were originally in, regardless of whetherthe column is at +V_(bias), or −V_(bias).

FIG. 5B is a timing diagram showing a series of row and column signalsapplied to the 3×3 array of FIG. 2 which will result in the displayarrangement illustrated in FIG. 5A, where actuated pixels arenon-reflective. Prior to writing the frame illustrated in FIG. 5A, thepixels can be in any state, and in this example, all the rows are at 0volts, and all the columns are at +5 volts. With these applied voltages,all pixels are stable in their existing actuated or released states.

In the FIG. 5A frame, pixels (1,1), (1,2), (2,2), (3,2) and (3,3) areactuated. To accomplish this, during a “line time” for row 1, columns 1and 2 are set to −5 volts, and column 3 is set to +5 volts. This doesnot change the state of any pixels, because all the pixels remain in the3-7 volt stability window. Row 1 is then strobed with a pulse that goesfrom 0, up to 5 volts, and back to zero. This actuates the (1,1) and(1,2) pixels and releases the (1,3) pixel. No other pixels in the arrayare affected. To set row 2 as desired, column 2 is set to −5 volts, andcolumns 1 and 3 are set to +5 volts. The same strobe applied to row 2will then actuate pixel (2,2) and release pixels (2,1) and (2,3). Again,no other pixels of the array are affected. Row 3 is similarly set bysetting columns 2 and 3 to −5 volts, and column 1 to +5 volts. The row 3strobe sets the row 3 pixels as shown in FIG. 5A. After writing theframe, the row potentials are zero, and the column potentials can remainat either +5 or −5 volts, and the display is then stable in thearrangement of FIG. 5A. It will be appreciated that the same procedurecan be employed for arrays of dozens or hundreds of rows and columns. Itwill also be appreciated that the timing, sequence, and levels ofvoltages used to perform row and column actuation can be varied widelywithin the general principles outlined above, and the above example isexemplary only, and any actuation voltage method can be used with thepresent invention.

The details of the structure of interferometric modulators that operatein accordance with the principles set forth above may vary widely. Forexample, FIGS. 6A-6C illustrate three different embodiments of themoving mirror structure. FIG. 6A is a cross section of the embodiment ofFIG. 1, where a strip of metal material 14 is deposited on orthogonallyextending supports 18. In FIG. 6B, the moveable reflective material 14is attached to supports at the corners only, on tethers 32. In FIG. 6C,the moveable reflective material 14 is suspended from a deformable layer34. This embodiment has benefits because the structural design andmaterials used for the reflective material 14 can be optimized withrespect to the optical properties, and the structural design andmaterials used for the deformable layer 34 can be optimized with respectto desired mechanical properties. The production of various types ofinterferometric devices is described in a variety of publisheddocuments, including, for example, U.S. Published Application2004/0051929. A wide variety of well known techniques may be used toproduce the above described structures involving a series of materialdeposition, patterning, and etching steps.

Data describing a monochrome display image may include one bit of dataper pixel. One embodiment of a monochrome display includes oneinterferometric modulator per pixel, the on or off state of themodulator being set based on the value of the one bit of data per pixel.A greyscale image may include several bits of data per pixel. Forexample, a “3 bit” grayscale display includes 3 bits of data per pixelthat correspond to one of eight shades of gray that may be assigned toeach pixel. One embodiment of a display for displaying an exemplary 3bit grayscale image includes three interferometric modulators for eachpixel. To obtain the eight shades, the three modulators reflect lightaccording to a ratio of 1:2:4. In one such embodiment, each of theinterferometric modulators includes mirrors having a reflective surfacearea that varies according to the ratio of 1:2:4. A particular shade ina pixel is obtained in such an embodiment by setting each modulator toan on or off state based on the binary value of a corresponding bit ofthe 3 bits of data. One embodiment of a color display works similarlyexcept that the color display includes a group of red, green, and blueinterferometric modulators. For example, in a 12-bit color display, 4 ofthe 12 bits correspond to each of 16 intensities of red, green, and bluewhich are produced by red, green, or blue interferometric modulators.Such greyscale or color displays have more display elements to addressthan does a monochrome display. In order to address these displayelements for such embodiments of gray or color displays, the number ofelectrical connections to the display control increases. For example, inone embodiment of a 3×3 3-bit grayscale display, each of the rows issubdivided into 3 subrows. Each pixel of such an embodiment of thedisplay comprises the interferometric modulators of the three subrows.One such embodiment has 3*3=9 row driver connections and 3 column driverconnections for a total of twelve driver connections rather than the 6for a 3×3 monochrome display.

One way of reducing the number of driver connections is to electricallyconnect together a group of modulators, for example, the 3 subrows inthe 3 bit grayscale embodiment discussed above, and drive the group witha signal that changes the state of a subset of the electricallyconnected group.

For example, one way to selectively address a group of electricallyconnected interferometric modulators is to apply a drive signal in apulse that is not of a sufficient duration to change the state of someof the group of modulators. Generally, the time period for a particularmodulator in the display to change state in response to the leading edgeof the row strobe may be referred to as a response time, τ. Note thatthe term “response time” may refer to either the time for aninterferometric modulator to move from a reflective to non-reflectivestate or the time for the modulator to move from the non-reflectivestate to the reflective state. In one embodiment of the interferometricmodulator display, the time period τ is conceptually the sum of anelectrical response time, τ_(RC), and a mechanical response time,τ_(M,). With respect to the electrical response time, each of theinterferometric modulators in the display is forms a respective circuitthat may be characterized by a resistor-capacitor (RC) time constant.The electrical response time, τ_(RC), is the time period from theleading edge of the row strobe pulse to the time that the circuit ischarged to the actuation or release voltage across the mirrors. Themechanical response time, τ_(M,) is the time period for the movablemirror to physically change positions once the actuation or releasevoltage has been reached. The time for the modulator to move to a newposition, τ_(M,) is dependent on factors such as a spring constantassociated with the movable mirror of the modulator and the resistanceto air of the mirror as it moves. One embodiment of the display includesgroups of electrically connected modulators with different responsetimes. By applying a common pulse to the group of modulators that isshorter than the response time of some of the modulators but longer thanthe response time of other of the modulators, the state of differentcombinations of the modulators can be set.

FIG. 7 is a partial schematic diagram of an embodiment of aninterferometric modulator display, similar to that shown in FIG. 5A, inwhich the rows have been subdivided into three subrows that share acommon driver connection. Each of the subrows defines oneinterferometric modulator at each column. As discussed above, if a rowstrobe is applied for a time that is less than the response time, themovable mirror of the interferometric modulator substantially maintainsits position. In the embodiment of FIG. 7, the interferometricmodulators of the subrows are depicted from top to bottom withdecreasing response times. In such an embodiment, the interferometricmodulators of the subrows may be addressed via the common driverconnection by suitably varying the duration of the row strobe so as tochange the state of only a selected portion of the subrows.

The response times of an interferometric modulator is affected by thecharacteristic resistor-capacitor (RC) time of the driver circuitinclusive of the modulator, i.e., the time for the movable mirror of themodulator to be charged to a particular voltage, of the mechanicalproperties of the modulator, and of the resistance of the movable mirrorto moving through air. In the embodiment depicted in FIG. 7, theresponse times of the interferometric modulators along the subrows arevaried by varying the series resistance of each subrow. Moreparticularly, in FIG. 7, each of the subrows is connected via a resistorthat provides a progressively lower resistance for each subrow from topto bottom. When a voltage is applied between the mirrors of themodulators, those with the larger resistors take longer to charge, andthus take longer for the voltage difference to fall outside of thestability window for sufficient time for the movable mirror to actuateto a new position.

FIG. 8 is a timing diagram that illustrates a series of row and columnsignals applied to the top row (Row 1) of the embodiment of the array ofFIG. 7 to produce the illustrated display arrangement. The row andcolumn signals in one embodiment are similar to those depicted in FIG.5B, except that a series of pulses is applied for each row to addresseach of the subrows, with each of the pulses varying in duration. Thereflective state of the display at the end of each line time isillustrated graphically on FIG. 7 below each the pulses of eachrespective line time. The pulses are applied during a series of linetimes for each row, one line time for each subrow. The row pulses foreach of these line times have magnitudes of +5 volts and a varying(decreasing from left to right) duration. The decreasing duration isselected so that the row pulses address only those modulators in subrowsthat have response times shorter than the row pulses.

The pulses of FIG. 8 set the state of the display to that depicted inFIG. 7 as follows. For the first line time for Row 1, Column 1, a columnpotential of −5 volts is applied along with the +5 volt row pulse to setthe state of the modulators of each of the subrows in the actuatedposition as illustrated along the bottom of FIG. 8. The Column 1potential remains at −5 for the remaining Row 1 lines times to maintaineach of the elements in the subrows in the actuated position. In Column2, a potential of +5 volts is applied in conjunction with the row pulsein the first line time to release all modulators in the subrows inColumn 2. During the second line time for Row 1, a Column 2 potential of−5 volts is applied in conjunction with the row pulse so as to actuatethe bottom two subrows of Row 1. The duration of the row pulses in thesecond line time is shorter than the response times of the top subrow,so the states of the modulator in the top subrow is maintained. Duringthe third row time for Row 1, a Column 2 potential is applied at −5volts in conjunction with the row pulse to actuate the modulator in thebottom subrow. Again, the row pulse duration of the third row time isshorter than the response time of the modulators in all but the bottomsubrow so that only the bottom row changes state. The data set forColumn 3 is applied according to FIG. 8 to set the state of the subrowsof Column 3.

In the illustrated embodiment, each of these line times for a row isapproximately the same. However, it is to be recognized that in otherembodiments, the line times may be shorter, for example, the line timesfor a row may shortened to correspond to the shorter row pulse durationsof each of the line times of a row. Further, any other suitable drivevoltage scheme may be used in place of the exemplary scheme depicted inFIGS. 5B and 7. Further, while the subrows in the illustrated embodimentinclude varying resistances that vary the RC time of the subrows, inother embodiments, the subrows may have varying capacitances,resistances, or a combination thereof.

In some embodiments, the response time of the interferometric modulatorsis varied by mechanical differences rather than electrical differencesin the subrows. For example, variation in the damping force caused bythe air in the small cavity between movable mirror and the fixed mirrorcan vary the response times. This damping force acts as a resistance tomoving the movable mirror through air. In one embodiment, this force isvaried by forming holes in the movable mirror to reduce the air pressureagainst the movable mirror as it actuates and thus changes theelectromechanical response of the actuator. In another embodiment, theholes are formed in the deformable film 34 of FIG. 6C. Other similarembodiments of interferometric modulators with varying response timesare discussed in U.S. patent application Ser. No. 10/794,737, filed Mar.3, 2004. In one embodiment, the response time of interferometricmodulators of the subrows varies based on variation of a combination ofone or more of the RC characteristic, the spring constant, or the airdamping force.

In other embodiments, other mechanical properties of the interferometricmodulators may be varied so as to vary the mechanical response times ofthe interferometric modulators between subrows. The response time isdependent on several factors that may be varied, including thethickness, mass, or material of the movable mirror 14 or the deformablelayer 34 of FIG. 6C. In some embodiments, interferometric modulators ineach of the subrows may have different spring constants. Embodiments mayalso vary the response times by varying the thicknesses, positions, orcomposition of the supports.

Rather than having varying response times, in other embodiments, theinterferometric modulators of each of the subrows may have varyingactuation and release voltages so as to enable a set of electricallyconnected subrows to be individually addressed. FIG. 9 is a diagram,similar to that of FIG. 3, of movable mirror position versus appliedpositive voltage illustrating an exemplary embodiment of threeinterferometric modulators that have respective nested stabilitywindows. The innermost nested hysteresis window, indicated by the traces802, has an actuation and release voltages having a magnitudes of 8 and4 volts, respectively. The next nested hysteresis window, indicated bythe traces 804, has an actuation and release voltages having amagnitudes of 10 and 2 volts, respectively. The outermost hysteresiswindow, indicated by the traces 804, has an actuation and releasevoltages having a magnitudes of 12 and 0 volts, respectively.

The hysteresis window of the modulators associated with each subrow maybe selected by varying the geometry and materials of the modulators. Inparticular, the width (difference between the actuation and releasevoltages), the location (the absolute values of the actuation andrelease voltages), and the relative values of the actuation and releasevoltages may be selected by varying geometric and material properties ofthe modulators. The varied properties may include, for example, thedistance between movable mirror supports, the mass associated with themovable mirror relative to the spring constant, the thickness, tensilestress, or stiffness of the mirror and/or the layers or mechanism thatmoves the mirror, the dialectic constant or thickness of a dielectriclayer between the stationary electrode and the movable electrode. Moredetails of the selection of the hysteresis properties of theinterferometric modulators are disclosed in U.S. Provisional Patent No.60/613,382, entitled “METHOD AND DEVICE FOR SELECTIVE ADJUSTMENT OFHYSTERESIS WINDOW,” filed on Sep. 27, 2004.

In one such embodiment, the interferometric modulators are arranged insubrows as in FIG. 8. The modulators of each of the subrows havehysteresis stability windows that are nested within each other. In theillustrated embodiment, the stability windows are nested from outer toinner, such as the windows depicted FIG. 9, from the top subrow to thebottom subrow. FIG. 10 is a timing diagram that illustrates a series ofrow and column signals applied to the first row (Row 1) of such anembodiment of an array to produce the illustrated display arrangement.The row and column signals in one embodiment are similar to thosedepicted in FIG. 8, except that the row pulses vary in magnitude ratherthan duration. The row pulses decrease in magnitude from left to right,corresponding to the subrows from top to bottom. This decreasingmagnitude of the pulses is selected to address only those modulators insubrows that have smaller actuation/greater release voltages. Forexample, in the illustrated embodiment, potentials of +6 and −6 voltsare applied to the columns and row pulses of 2, 4, 6 volts are appliedto the row.

The pulses of FIG. 10 set the state of the display to that depicted inFIG. 7 as follows. For the first line time for Row 1, Column 1, a columnpotential of −6 volts is applied along with the +6 volt row pulse to setthe state of the modulators of each of the subrows in the actuatedposition as illustrated along the bottom of FIG. 8. The Column 1potential remains at −6 for the remaining Row 1 line times to continueto set the state of each of the elements in the subrows in the actuatedposition. In Column 2, a potential of +6 volts is applied in conjunctionwith the row pulse at +6 volts in the first line time to release allmodulators in the subrows in Column 2. During the second line time forRow 1, a Column 2 potential of −6 volts is applied in conjunction with arow pulse of +4 volts so as to actuate the bottom two subrows of Row 1.During the third row time for Row 1, a Column 2 potential is applied at−6 volts in conjunction with the row pulse of +2 volts to actuate themodulator in the bottom subrow. The set of pulses for Column 3 isapplied according to FIG. 8 to set the state of the subrows of Column 3.

FIG. 11 is a flowchart illustrating one embodiment of a method 850 ofupdating an embodiment of a display such as in FIGS. 6 and 9A. Themethod 850 begins at a block 852 in which the driver 22 of FIG. 22receives image data value for a subrow. In one embodiment, the driver 22receives the data value from a frame buffer. Next at a block 854, thedriver 22 applies a row strobe to all subrows of interferometricmodulators along with a column potential that corresponds to the imagedata value. Moving to block 856, the driver 22 receives the data for thenext subrow. Next at block 860, the acts of blocks 854 and 856 arerepeated for each of the subrows. In one embodiment, the acts of theblocks 854 and 856 occur at least partially concurrently.

It is to be recognized that while certain embodiments disclosed hereinare discussed with respect to “rows” and “columns,” these terms are usedfor convenience only in describing these embodiments. In otherembodiments, the properties attributed to rows or columns in theexemplary embodiments may be completely or partially reversed as wouldbe apparent to one of skill in the art. Further, while embodiments areillustrated in FIGS. 7 and 9B with respect to one particular drivescheme, any other suitable drive scheme may be adapted to vary theduration or magnitude of the applied pulses in accordance with thedisclosed invention. In addition, while in one embodiment, the groups ofinterferometric modulators that share a common driver connection arearranged in subrows, it is to be recognized that other embodiments mayinclude any arrangement of groups of interferometric modulators.

Moreover, while certain embodiments have been discussed with respect toelectrically connected groups of interferometric modulators withdifferent response times, and certain other embodiments discussed withrespect to electrically connected groups of interferometric modulatorswith different hysteresis stability windows, other embodiments mayinclude groups of electrically connected modulators that have differentresponse times and different hysteresis stability windows. Suchembodiments may be addressed using a series of pulses that vary in bothduration and voltage.

While the above detailed description has shown, described, and pointedout novel features of the invention as applied to various embodiments,it will be understood that various omissions, substitutions, and changesin the form and details of the device or process illustrated may be madeby those skilled in the art without departing from the spirit of theinvention. The scope of the invention is indicated by the appendedclaims rather than by the foregoing description. All changes which comewithin the meaning and range of equivalency of the claims are to beembraced within their scope.

1. A method of addressing a plurality of display elements having atleast a first and second display element and characterized by respectiveresponse thresholds, the method comprising: generating a first pulsecharacterized by a parameter having a value greater than the responsethreshold of the first display element and less than the responsethreshold of the second display element; and applying the first pulse tothe plurality of display elements.
 2. The method of claim 1, wherein theparameter comprises a pulse duration and the response thresholdcomprises a response time of the respective display element.
 3. Themethod of claim 1, wherein the parameter comprises a voltage magnitudeand the response threshold comprises an actuation voltage of therespective display element.
 4. The method of claim 1, wherein the stateof the second element is maintained.
 5. The method of claim 1, furthercomprising: receiving an image data signal; and setting the state ofeach of the first and second display elements based, at least in part,on the image data signal.
 6. The method of claim 1, further comprising:generating a second pulse of a second value of the parameter that isgreater than the response threshold of a third display element, whereinthe second value is less than the response threshold of the firstdisplay element; and applying the second pulse to the plurality ofdisplay elements.
 7. The method of claim 1, wherein applying the firstpulse to the plurality of display elements comprises applying the firstpulse to a plurality of interferometric modulators.
 8. The method ofclaim 7, wherein each of the plurality of interferometric modulatorscomprises a movable mirror characterized by a respective resistance whenmoving through air.
 9. The method of claim 8, wherein the respectiveresponse times of the plurality of interferometric modulators are based,at least in part, on the respective air resistance of the movablemirror, and wherein the first and second display elements arecharacterized by different resistances to movement through air.
 10. Themethod of claim 1, wherein the plurality of display elements ischaracterized by respective resistor-capacitor (RC) time constants. 11.The method of claim 10, wherein the respective response times of theplurality of display elements are based, at least in part, on therespective RC time constants, and wherein the first and second displayelements are characterized by different RC time constants.
 12. Themethod of claim 1, wherein the plurality of display elements ischaracterized by respective physical properties.
 13. The method of claim12, wherein the respective response times of the plurality of displayelements are based, at least in part, on the respective springconstants, and wherein the first and second display elements arecharacterized by different spring constants.
 14. The method of claim 1,wherein applying the first pulse to the plurality of display elementscomprises applying the first pulse to a pixel of a visual display. 15.The method of claim 14, wherein the pixel comprises the plurality ofdisplay elements.
 16. The method of claim 1, wherein applying the firstpulse to the plurality of display elements comprises applying a voltagepulse.
 17. The method of claim 1, wherein applying the first pulse tothe plurality of display elements comprises applying a first voltagepulse to a row of the display elements and applying a second voltagepulse to a column of the display elements, and wherein each of the firstand second display elements is associated with the row and the column.18. A driver circuit for addressing a plurality of display elementshaving at least a first and second display element and characterized byrespective response thresholds, the driver circuit comprising: means forgenerating a first pulse characterized by a parameter having a valuegreater than the response threshold of the first display element andless than the response threshold of the second display element; andmeans for applying the first pulse to the plurality of display elements.19. The driver circuit of claim 18, wherein the parameter comprises apulse duration and the response threshold comprises a response time ofthe respective display element.
 20. The driver circuit of claim 18,wherein the parameter comprises a voltage magnitude and the responsethreshold comprises an actuation voltage of the respective displayelement.
 21. The driver circuit of claim 18, wherein the state of thesecond element is maintained.
 22. The driver circuit of claim 18,further comprising: means for receiving an image data signal; and meansfor setting the state of each of the first and second display elementsbased, at least in part, on the image data signal.
 23. The drivercircuit of claim 18, further comprising: means for generating a secondpulse of a second value of the parameter that is greater than theresponse threshold of a third display element, wherein the second valueis less than the response threshold of the first display element; andmeans for applying the second pulse to the plurality of displayelements.
 24. The driver circuit of claim 18, wherein the means forapplying the first pulse to the plurality of display elements comprisesmeans for applying the first pulse to a plurality of interferometricmodulators.
 25. The driver circuit of claim 18, wherein the means forapplying the first pulse to the plurality of display elements comprisesmeans for applying the first pulse to a pixel of a visual display. 26.The driver circuit of claim 18, wherein the means for applying the firstpulse to the plurality of display elements comprises means for applyinga voltage pulse.
 27. The driver circuit of claim 18, wherein the meansfor applying the first pulse to the plurality of display elementscomprises means for applying a first voltage pulse to a row of thedisplay elements and applying a second voltage pulse to a column of thedisplay elements, and wherein each of the first and second displayelements is associated with the row and the column.